The present invention relates in general to brushless motor and, more particularly, to a small brushless motor having a skeleton type stator core.
In general, small motors of the skeleton type having a stator core and a stator coil, such as, for example the shaded-pole motor as disclosed in Japanese Patent Publication No. 57-16575 published Apr. 16, 1982, are small and simply constructed, and have been used usually because of their moderate price owing to the ease of manufacture. The shaded-pole motor has, in addition to the stator coil, shading coils wound on the stator core for producing a rotational magnetic field. In this motor, since a large part of the stator electric current flows through the shading coils, the efficiency of the motor is decreased correspondingly. In other words, the use of shading coils hinders the efficient operation of the motor. Usually, motors are driven by, A.C. electrical power supply, typically by commercial A.C. power. The rated speed of the motor, therefore, is determined by the frequency of the commercial power. In order to obtain different rated speeds, therefore, it has been necessary to employ a complicated control circuit or to design motors for different rated speeds.
It is to be also noted that the configuration of the air gap between the stator and the rotor, defined by the configuration of the cross-section of a rotor bore in the core, is uniform in the skeleton-type, small motor disclosed in Japanese utility Model Publication No. 57-8368 published Feb. 17, 1982.
Therefore, the rotational position at which the rotor is stopped after the power supply is turned off is not constant due to various factors such as the inertia of the load connected to the motor, friction of the bearing, and so forth. This does not cause any substantial problem in the shaded-pole type motor. However, in a brushless motor having a rotor constituted by a permanent magnet, the rotor stopping position induces a certain problem in regard to the re-starting of the motor. More specifically, in this type of motor, it is necessary to switch over the polarity of the power supply in accordance with the rotational position of the rotor when the motor is to be re-started. Therefore, a Hall generator has been used for detecting the position of the rotor. Furthermore, when the rotor and the stator have two poles, respectively, it is difficult to re-start the motor when the rotor is stopped at a rotational position where the centers of the magnetic flux through the rotor bore and rotor poles substantially aligned with each other. This problem would be solved by adding Hall generators, stator poles and rotor poles in such a way as to meet a given condition. For instance, it is possible to obviate the above-described problems by using, in combination, a stator having three poles and a rotor having four poles or two poles. This, however, requires a large number of Hall generators, which in turn raises the cost of production of the electric motor through its complication and an increase in the size of the power supply control circuit. Such a complicated control circuit is difficult to maintain and tends to degrade the reliability of the motor.
In order to solve these problems, various measures have been proposed to make the rotor stop at a constant rotational position. One of these known measures is to provide a non-uniform air gap between the rotor and the stator by, for example, increasing the air gap at a portion of the circumference around the rotor. Unfortunately, however, this countermeasure cannot perfectly ensure the stopping of the rotor at a constant position due to the inertia of the rotor.
Japanese Patent Application No. 55-151204 (published May 24, 1982 under No. 57-83149) discloses an electric motor in which the bore of the cylindrical stator core is made to have an irregular form so as to provide a non-uniform air gap, thereby ensuring the stopping of the rotor at a constant rotational position. In this case, however, it is necessary to wind the stator coil directly on the stator core at a uniform thickness and to position this coil precisely such that the stator coil produces a magnetomotive force at an angle of 90.degree. to the direction of the magnetic field produced by the rotor. In addition, the mean air gap is increased by an amount corresponding to the thickness of the stator coil. The increased air gap correspondingly increases the leakage of magnetic flux and reduces the working efficiency of the motor. To avoid this problem, it is necessary to employ a permanent magnet which has a high magnetic flux density and which is strongly resistant to irreversible demagnetization. This magnet, however, increases the cost of the motor. Furthermore, since the minimum air gap is limited due to the thickness of the stator coil, the stopping torque for limiting the rotation of the rotor towards the constant stopping position is rather small. This in turn limits the torque of the load which is to be driven by the motor.
On the other hand, Japanese Patent Application No. 53-101072 (published Mar. 1, 1980 under No. 55-29246) discloses single phase brushless motor which comprises: a field system having a field coil; a rotor constituted by a permanent magnet rotatably disposed in the field; magnetic wires disposed under the influence of the N and S poles of the rotor and constituted by a core and a shell having different levels of coercive forces, the magnetic wires being magnetized in the axial direction; a coil adapted to produce a voltage in response to a change in the magnetic field of the magnetic wires; and an electric circuit for controlling the power supply to the field coil in response to the voltage signal generated in the coil.
In the conventional brushless motor mentioned above, the magnetic wires arranged in a pair are used as a sensor for detecting the rotational position of the rotor. Unfortunately, however, it is difficult to mount these sensors. In some cases, the timing of switching over the power supply to the stator coil varies so as to cause various problems such as unstable rotation of the rotor and a reduction in the efficiency of the motor. In addition, the use of a plurality of position sensors makes it difficult to reduce the size and cost of the motor.
Furthermore, in the known small motors mentioned hereinbefore, the stator core has to be exactly cylindrical and, in addition, the stator coil has to be divided into two sections, and therefore the overall structure of the motor becomes complex.